This project will take advantage of a unique opportunity to develop targeted therapy for thyroid cancer. We and others have found that the ras/raf/MEK signal transduction pathway is activated by specific genetic events in almost all cases of papillary thyroid cancer, and we have recently shown that thyroid cancer cell lines are remarkably sensitive to MEK inhibition. In addition, MEK inhibition leads to upregulation of a group of thyroid-specific genes necessary for iodine uptake. In this project, we will pursue these findings to develop and translate optimal strategies for use of MEK inhibitors, alone and in combination, for therapy of thyroid cancer. Sensitivity of thyroid cancer cell lines to AZD6244, a novel MEK inhibitor, will be evaluated both in cell culture and in xenografts, and correlated with BRAF and other mutation status. The potential role of the PI3K pathway as an escape pathway from MEK inhibition will be evaluated. Potential molecular markers for efficacy of the drug will be developed, To develop combination therapy approaches, signal transduction pathways that are activated in thyroid cancer cells will be evaluated as potential therapeutic targets. Initially, we will concentrate on the PI3K signal transduction pathway, since it has been shown to be activated in most cases of thyroid cancer; additional promising pathways will also be evaluated. A potential mutation, conferring resistance to AZD6244 in a thyroid cancer cell line, will be explored. Such a mutation could indicate inherent resistance to AZD6244, or could be a potential mechanism for development of resistance during AZD6244 treatment. The ability of MEK inhibition, alone and in combination, to induce biologically relevant reactivation of iodine uptake will be assessed in cell culture and in xenograft models. These studies should provide important molecular information on the effects of this novel MEK inhibitor on differentiation in thyroid tumor cells, and will form the basis for future clinical trials of these drugs on thyroid cancer patients. Relevance: There are approximately 26,000 new cases of thyroid cancer per year in the United States, a 3- fold increase in the past three decades. In approximately 10% of these cases there is recurrent disease that is refractory to radioiodine and other therapy, with substantial mortality. Our studies to develop targeted therapy for thyroid cancer, will address this unmet medical need.